The presence of large deposits of oil shale in the Rocky Mountain regions of the United States have given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit and including dolomite with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598, and 4,118,701. These patents are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented, permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method for forming an in situ oil shale retort as described in U.S. Pat. No. 4,043,595 includes excavating a first portion of the formation from within the boundaries of the in situ oil shale retort being formed to form a void, where the surface of the formation defining the void provides at least one free face extending through the formation within the boundaries. A second portion of the formation is explosively expanded toward the void to form an in situ oil shale retort containing a fragmented permeable mass of formation particles. The fragmented permeable mass in the retort has a void fraction which is equal to the ratio of the volume of the void to the combined volume of the void and the space occupied by the second portion of the formation. As used herein the term "void fraction" refers to the ratio of the volume of the voids or spaces between particles in the fragmented mass to the total volume of the fragmented permeable mass of particles in an in situ oil shale retort. For example, in a fragmented mass with a void fraction of 20%, 80% of the volume is occupied by particles, and 20% is occupied by the spaces between particles.
One method for supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a primary combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as an oxygen-containing gaseous primary combustion zone feed to advance the primary combustion zone through the retort. In the primary combustion zone, oxygen in the primary combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the retort, the primary combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the primary combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the primary combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products, including gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process, is also withdrawn from the bottom of the retort.
When preparing an in situ oil shale retort, the void fraction may not be uniform throughout the entire fragmented mass. For example, the fragmented mass can contain two fluid flow paths between the inlet and outlet of the retort, where one of the fluid flow paths has a relatively lower flow resistance than the other fluid flow path. When processing the fragmented mass to recover shale oil, there is a tendency for gas introduced to the retort to channel along the flow path of relatively lower flow resistance. This channelling can result in a warped combustion zone, where a portion of the combustion zone advancing along the flow path of relatively lower flow resistance is farther advanced than a portion of the combustion zone advancing along the flow path of relatively higher flow resistance.
This is undesirable because it is found that the best yield of shale oil from oil shale is obtained when the primary combustion zone moves through the retort as a substantially flat zone which is substantially uniformly perpendicular to its direction of advancement. When the primary combustion zone is skewed and/or warped some of the shale oil produced may be burned, thereby reducing the total yield. In addition, with a skewed and/or warped primary combustion zone, excessive cracking of hydrocarbon products produced in the retorting zone can result. It is, therefore, desirable to have the primary combustion zone progress through the fragmented mass as a substantially flat horizontal wave.
Therefore, there is a need for a method for operating an in situ oil shale retort having fluid flow paths of different gas flow resistance where the primary combustion zone is advanced through the fragmented mass as a substantially flat zone which is substantially uniformly perpendicular to its direction of advancement.